If you have a Bemis True HEPA air purifier, you are using an extremely effective air purification system that employs filter technology first developed for the American space program. The Bemis True HEPA 1101 filter is known to remove 99.97% of all airborne particles. True HEPA filters are used in environments where the highest air purity is demanded, like high-tech laboratories and hospital operating rooms. This 1101 filter is designed to work in concert with the carbon pre-filter (#1102) to establish the best air purification available. As with any filter (if it's doing its job) it will capture particles until it has reached its capacity to do so. Once a filter is full or nearly full, its efficiency and usefulness is greatly diminished, and so is that of your air purifier. This means it is time to replace the air filter 1101 with a fresh one - and it is always best to use genuine Bemis Essick Air filters that are designed and intended for use in your air cleaner. As a matter of fact, your Bemis True HEPA Air Purification System features their exclusive FilterWatch Sensor. This sensor accurately monitors your filter's condition. When one or both of the filters become blocked, the sensor alerts you with a distinct red light - giving a positive notification that it is time for filter replacement. This pair of genuine Bemis True HEPA 1101 Replacement air filters will restore optimum efficiency to your air purifier.